Selective catalytic reduction exhaust after-treatment

ABSTRACT

A method and system for selective catalytic reduction of NOx emitted by a vehicle engine combines hydrogen from an on-board hydrogen storage unit with engine exhaust gas and then contacts the combined hydrogen and exhaust gas with a NOx catalyst. The on-board hydrogen storage unit is additionally used as at least one of a fuel for the engine and a source of chemical potential energy for charging an auxiliary power unit.

FIELD OF THE INVENTION

The present invention relates to improved methods and systems for selective catalytic reduction of nitrogen oxides (NOx).

BACKGROUND OF THE INVENTION

The invention relates to the use of certain catalytic systems for reducing nitrogen oxides (NOx) and to a corresponding process. Nitrogen oxides primarily come from combustion off-gases, in particular from internal-combustion engines such as diesel engines, Otto engines, and Daimler-Otto engines.

Many solutions have been proposed for abating NOx emissions of motor vehicle engines. Among these are non-selective catalytic reduction, such as may be performed in a standard three-way catalytic converter unit. The most common reactions that are utilized convert NOx, especially NO and NO₂, to N₂ and O₂.

These non-selective catalytic reduction solutions have been further supplemented, and in some cases replaced with, selective catalytic reduction (SCR) technologies. SCR most commonly involves the use of a chemical reductant that co-reacts with NOx to effect conversion to N₂ and O₂, and in some cases H₂O. The most common SCR technologies used in automotive vehicles employ ammonia or urea as the reductant, with either an ammonia reservoir or an ammonia precursor supply, e.g., a urea supply, available to provide the ammonia, or the urea. This technology, too, has been shown capable of reducing NOx emissions.

However, still further improvement in environmental performance of automotive engines is desirable, both from an environmental and from an economic perspective. This particularly includes further decrease in NOx generation, as well as improvement in degradation of NOx generated, by internal combustion engines. It would also be desirable to eliminate the need for on-board vehicle storage of ammonia and/or urea chemicals.

SUMMARY OF THE INVENTION

The present invention provides an improved selective catalytic reduction system for reduction of NOx from automotive vehicle engine exhaust gas using hydrogen as a reductant. The present invention provides the use of hydrogen for selective catalytic reduction of NOx to N₂, H₂O, and optionally O₂, the NOx being that generated by an automotive vehicle engine, and the hydrogen being that stored on-board the vehicle for use as at least one of (a) a fuel for the engine or (b) a source of chemical potential energy for charging an auxiliary power unit. The present invention further provides:

Processes for selective catalytic reduction of NOx generated by an automotive vehicle engine, including steps of (A) providing hydrogen from an on-board hydrogen storage unit of an automotive vehicle in which the hydrogen is being stored for use as at least one of (1) a fuel for the engine or (2) a source of chemical potential energy for charging an auxiliary power unit, (B) combining the hydrogen with exhaust gas released from the engine to form a hydrogen-exhaust gas combination, and (C) contacting the gas combination with a selective catalytic reduction NOx catalyst under conditions in which the catalyst can catalyze the selective catalytic reduction of NOx to N₂, H₂O, and optionally O₂;

Automotive vehicle systems for selective catalytic reduction (SCR) of NOx generated by an automotive vehicle engine of a vehicle in which hydrogen is being stored for use as at least one of (a) a fuel for the engine or (b) a source of chemical potential energy for charging an auxiliary power unit, the systems having: (A) an SCR NOx catalyst, (B) an SCR hydrogen distribution system capable of delivering hydrogen from the on-board hydrogen storage unit to the exhaust gas stream at a point upstream from said SCR NOx catalyst, said hydrogen distribution system including a hydrogen injector therefore, and (C) an SCR control unit (SCR CU), the SCR CU being operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen directly proportional to (1) the amount of exhaust gas generated per unit time by the engine or (2) the amount of NOx generated per unit time by the engine;

Such systems having a NOx sensor capable of sensing the concentration of NOx present in the engine exhaust, the SCR CU being operative to deliver to the exhaust stream an amount of hydrogen proportional to an input received from the NOx sensor;

Such systems in which the SCR CU is operative to control the temperature of the SCR NOx catalyst; such systems having a NOx catalyst temperature sensor, the SCR CU being operative to control the temperature of the catalyst based on an input received from the NOx catalyst temperature sensor;

Such systems in which the SCR CU is operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen that is approximately stoichiometric with the NOx concentration in the engine exhaust;

Motorized vehicles having a fueled engine and an on-board hydrogen supply, in which the vehicle includes such an SCR NOx reduction system; such vehicles in which the engine is a diesel engine and the on-board hydrogen supply is operative to provide hydrogen for an on-board hydrogen-sourced auxiliary power supply; such vehicles in which the engine is an internal combustion engine configured to use hydrogen as a supplemental fuel and the on-board hydrogen supply is operative to provide hydrogen as a supplemental fuel; and such vehicles in which the engine is an internal combustion engine configured to use hydrogen as its main or sole fuel source and the on-board hydrogen supply is a hydrogen fuel reservoir operative to provide hydrogen as the main or sole fuel.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating one embodiment of a SCR system for reduction of NOx generated by a Daimler-Otto engine configured to utilize hydrogen as a fuel supplement, and depicting a control unit capable of controlling hydrogen delivery for SCR of exhaust gases and optionally of controlling SCR NOx catalyst bed temperature; and

FIG. 2 is a diagram illustrating one embodiment of a SCR system for reduction of NOx generated by a diesel engine in a vehicle having a hydrogen-based auxiliary power unit, and depicting a control unit capable of controlling hydrogen delivery for SCR of exhaust gases and optionally of controlling SCR NOx catalyst bed temperature;

FIG. 3 is a diagram illustrating one embodiment of a SCR system for reduction of NOx generated by an engine utilizing hydrogen as its sole combustible fuel, and depicting a control unit capable of controlling hydrogen delivery for SCR of exhaust gases and optionally of controlling SCR NOx catalyst bed temperature.

DETAILED DESCRIPTION

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

SCR is a very effective way to reduce NOx in exhaust gases emitted by engines operating at a lean ratio, and to reduce NOx in an O₂-rich environment. Typically, urea or ammonia is used to reduce NO to N₂ and O₂. However, in some diesel applications, ongoing work is aimed at replacing the vehicle battery and charging system with a hydrogen fuel cell. In other applications, hydrogen is being considered as a supplemental fuel used in admixture with a hydrocarbon or other primary fuel for an internal combustion engine (ICE) because it reduces CO₂ and other emissions. In both cases, a catalytic converter can be necessary to reduce NOx to mandated levels.

However, using the hydrogen already stored on board, as a reductant for NOx would offer a number of advantages over existing NOx reduction technologies. For example, it would eliminate one on-board reservoir, in the above-described diesel case; and it would ensure extremely low cycle NOx emissions, in the hydrogen-supplemented ICE case described above. In addition, in comparison to the use of an ammonia or urea reductant, hydrogen provides significantly greater diffusivity, thereby increasing the uniformity of the reductant-exhaust gas mixture in the limited time available in the gas stream between the reductant entry point and the SCR NOx catalyst. This increased uniformity of mixing offers an increased NOx reduction efficiency and increased reductant utilization rate in the catalyst chamber.

Use of hydrogen as reductant also avoids the problem of “ammonia slip” that can occur with use of ammonia or urea, in which unutilized reductant passes through the catalyst chamber. This can produce increased level of environmentally undesirable particulate matter, such as ammonium nitrates and ammonium sulfates; and in some states, commercial and vehicular ammonia release is regulated as a toxic emission. In some cases, this has required the use of a further ammonia oxidation catalyst downstream from the SCR NOx catalyst, in order to remove ammonia from the treated exhaust. It is believed that replacing an ammonia or urea reductant with hydrogen would both decrease the amount of incidence of reductant “slip” and avoid or mitigate the need for a downstream reductant oxidation catalyst.

In the case of a hydrogen fueled ICE running lean, so as to produce high concentrations of O₂ in the exhaust, which disfavors reduction of NOx, there is no current SCR after-treatment technology that does not require urea or ammonia, which requires its own on-board fluid system. For a diesel engine-powered vehicle with an hydrogen-supplied auxiliary power unit (APU), SCR is the only technology known effective for NOx reduction, yet this still requires the presence of the other on-board fluid, ammonia or an ammonia source, such as urea.

Elements of an SCR system according to the present invention include the following, with reference to FIGS. 1, 2, and 3, arranged along an engine exhaust gas flow path.

A fuel source 1 is coupled to an engine 3 which generates an exhaust stream 4. An outboard hydrogen storage unit 2 is coupled to SCR hydrogen distribution system 7. Storage unit 2 is additionally used as a fuel source for a Daimler-Otto engine 3 of FIG. 1 or engine 3 of FIG. 3 or as a source of chemical potential energy for charging an auxiliary power unit 10 of FIG. 2.

SCR hydrogen distribution system 7 is capable of delivering hydrogen from the on-board storage unit 2 (FIGS. 1 and 2) or from hydrogen fuel reservoir 1 (FIG. 3) to exhaust gas stream 4 via hydrogen injector 7 a. The flow of such hydrogen is regulated by one or more valves 7 b.

SCR control unit (SCR CU) 6 is operable to control the valves 7b of distribution system 7 to permit a desired amount of hydrogen to be imported into exhaust stream 4 and, optionally, to control temperature of SCR NOx catalyst 8. SCR CU 6 may comprise a separate unit or could be a part of the vehicle's engine control unit.

Optimally, NOx sensor 5 is coupled as an input to SCR CU 6 for use in enabling SCR CU 6 to appropriately determine a desired amount of hydrogen and the time of delivery of the desired amount to exhaust stream 4.

Also, optionally, SCR NO_(x) catalyst temperature sensor 8 a is coupled for input to SCR CU 6 for use in controlling a heating element of temperature regulator 8 b for catalyst 8.

Other typical elements depicted in FIGS. 1 and 2 include: fuel 1, engine 3 (a Daimler-Otto engine in FIG. 1; a Diesel engine in FIG. 2), exhaust gases 4, treated exhaust 9, and (in FIG. 2) a hydrogen-powered auxiliary power unit 10. In the on-board hydrogen storage unit 2 (in FIGS. 1 and 2) and in the hydrogen fuel reservoir 1 (in FIG. 3), hydrogen is typically stored as diatomic hydrogen (H₂), but other forms of hydrogen may also be employed, e.g., atomic hydrogen or metal-hydride- or metal-absorbed or -adsorbed hydrogen species.

A Daimler-Otto internal combustion engine 3 as depicted in FIG. 1 will typically use an organic fuel 1. As used herein, the term “organic fuel” means carbon and combustible carbon compounds including, e.g., homo- and hetero-hydrocarbons (e.g., alcohols, fatty acids, esters, ethers and oxacycloalkanes, and the like), carbohydrates, and carbon monoxide; and organic fuel mixtures.

An SCR system according to the present invention will include a SCR NOx catalyst to catalyze the reaction between hydrogen and NOx. Any catalyst known effective in the art for NOx SCR may be utilized. Common examples of transition metal oxide catalysts therefor include Fe₂O₃, V₂O₅, MoO₃, WO₃, Cr₂O₃, and Mn₂O₃. Common examples of metal catalysts therefore include Ag, Co, Cu, Fe, Ni, and the nobel metals, e.g., Au, Pt, Pd, Ir, Ru, Rh, and Os. Zeolite catalysts are also useful herein and include the metal ion-exchanged zeolites.

One or more than one catalyst may be used, either mixed together, or located in or on separate zones of the catalyst bed. The catalysts are typically formulated with other materials to form an active catalyst layer. Representative examples of other materials for this purpose include TiO₂, Al₂O₃, and SiO₂, and combinations thereof. Thus, in one embodiment, the active catalyst may be a TiO₂—V₂O₅—WO₃ combination.

Preferably the catalyst bed will comprise a solid support for the active catalyst. Any support materials known effective in the art for supporting NOx SCR catalysts may be used. In one embodiment, the support will be a metallic, ceramic, silica, alumina, or zeolite material.

The catalyst bed may take any format known effective in the art as effective for effluent gas treatment. Examples include parallel plate formats, in which the exhaust stream is shaped into substantially planar flow paths of wide, thin rectangular cross section, and parallel tube or tunnel formats, in which the exhaust stream is shaped into flow paths having at least substantially triangular, square, pentagonal, hexagonal, octagonal, and/or circular cross-section. Typical internal diameters or plane thicknesses for flow paths range from about 2 to about 20 mm; preferable dimensions may range, e.g., from about 5 to about 20 mm. In some preferred embodiments, such a dimension will fall within a range of about 6 to about 15 mm; in some preferred embodiments, such a dimension will fall within a range of about 8 to about 12 mm.

The parallel plate, tube, or tunnel passage formats are typically, and preferably, arranged in arrays of parallel passages, with the flow path direction through the passages being substantially orthogonal to the plane of the array. Typically, and preferably, multiple such arrays are present in the SCR NOx catalyst unit, spaced one from the next, and situated across the exhaust gas stream, which is thereby forced through the arrays via their parallel passages.

The SCR NOx catalyst unit may be heated by a heating element to which electric power is distributed by the engine control unit (ECU). A temperature sensor may also be present in or on the SCR NOx catalyst unit and may be connected to the ECU in order to permit the ECU to monitor the temperature of the unit and to vary the level of power provided to the heating element in order to maintain the temperature of the unit within an optimal range for NOx reduction. In one preferred embodiment, the temperature will be maintained within a range of, or alternatively will be maintained at an approximately constant temperature that is within a range of, about 180° to about 600° C.

In one preferred embodiment, the active catalyst will be or will include at least one nobel metal catalyst, and the temperature will be maintained at or within a range of about 180° to about 300° C. In one preferred embodiment, the active catalyst will contain Fe₂O₃, V₂O₅, MoO₃, and/or WO₃, and the temperature will be maintained at or within range of about 230° and about 430° C. In one preferred embodiment, the catalyst will be or will include a metal ion-exchanged zeolite catalyst and the temperature will be maintained at or within range of about 360° to about 600° C. Non-limiting examples of zeolite catalysts useful for this purpose include, e.g.: platinum, copper, or cobalt cation-exchanged zeolites, and copper-mixed cation-exchanged zeolites, e.g., copper and cobalt, or copper and osmium cation-exchanged zeolites.

A system according to the present invention may also include a NOx trap, and this may optionally be one that can be selectively operated to release adsorbed or absorbed NOx when the SCR catalyst has the capacity available to reduce them. The NOx trap may be a NOx trap material that can be incorporated into the SCR catalyst unit or even admixed with the active catalyst itself. Any of the various NOx traps may be utilized and illustrative examples of these include, e.g., alkali metal NOx storage components, such as potassium, present in the form of a salt or alloy with at least one further metal or as an oxide.

A system according to the present invention will include an SCR hydrogen distribution system 7, of which one portion is a hydrogen injector 7 a though which hydrogen can pass from the on-board hydrogen storage unit or hydrogen fuel reservoir and into the exhaust stream. The hydrogen injector 7 a preferably is, or is operated in concert with, a valve that can regulate the amount of hydrogen flowing through the injector and into the exhaust stream.

An SCR system according to the present invention will also comprise an SCR control unit (SCR CU), which may be at least a portion of an engine control unit (ECU), i.e. an electronic logic circuitry module that can store and implement control programs for opening and closing valves, transmitting electrical power, and the like, optionally based on sensor inputs received from sensors. In one embodiment, the portion of the ECU operating SCR-related systems is referred to as the SCR-ECU. The SCR CU (or SCR ECU) will be configured to be capable of operating the SCR hydrogen distribution system so as to deliver to the exhaust stream an amount of hydrogen directly proportional to at least one of (1) the amount of exhaust gas generated per unit time by the engine or (2) the amount of NOx generated per unit time by the engine. In performing this task, the SCR CU optionally utilizes an input or inputs received from an optional NOx sensor capable of sensing the concentration of NOx overall, or capable of sensing distinct species of NOx, preferably at least NO and NO₂, in the engine exhaust, to thereby better control the amount of hydrogen released into the exhaust stream. Alternatively, a sensor capable of transmitting a signal indicating the volume of exhaust gas generated per unit time, may provide an input or inputs to the SCR CU for this purpose. In one embodiment, the amount of hydrogen delivered to the exhaust stream will be at least approximately stoichiometrically proportional to the amount of NOx generated per unit time by the engine. Such an alternative sensor may detect one or more engine operating conditions such as, e.g., engine temperature, cycle rate, fuel composition, oxygen concentration, and so forth.

The SCR CU optionally includes a control program capable of maintaining and/or regulating the temperature of the SCR NOx catalyst bed, e.g., as by controlling the amount of heating provided by an optional SCR NOx catalyst bed heating element. The NOx catalyst bed temperature control program can optionally utilize an input or inputs from an optional NOx catalyst temperature sensor, to better control the catalyst temperature.

With reference to the Figures, an SCR CU (or SCR ECU) 6 according to the present invention, or the ECU of which it is a part, will preferably include a microcomputer with a control program that controls the amount of hydrogen delivered to the exhaust stream, and optionally with a control program that controls the temperature of the SCR NOx catalyst bed. The control unit can also include other conventional components such as an input interface circuit(s), an output interface circuit(s), and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. The memory circuit stores processing results and control programs that are run by the processor circuit. The control unit is operatively coupled to the other components of the hydrogen distribution system, and optionally the SCR NOx catalyst bed temperature regulating system, in any conventional manner. As illustrated in one embodiment, the input(s) received by the input circuits may be obtained from an optional exhaust gas NOx sensor 5 and/or from an optional SCR NOx catalyst bed temperature sensor 8 a.

The input(s) that the SCR CU 6 receives from an optional NOx sensor 5 (or an exhaust gas concentration detector as an alternative version thereof) located upstream of the SCR NOx catalyst 8, will be a signal that provides a direct or indirect indication of the amount of NOx present in the exhaust stream per unit time. In any embodiment in which the SCR CU 6 receives input(s) directly or indirectly indicating the exhaust gas NOx concentration, these will provide an indication of whether or not the amount or rate of hydrogen delivered to the exhaust gas stream should be maintained or modulated, in accordance with the SCR CU control program.

The input(s) that the SCR CU 6 receives from an optional SCR NOx catalyst bed temperature sensor 8 a will be a signal that provides a direct or indirect indication of the temperature of the catalyst bed. As an alternative or addition to an SCR NOx catalyst bed temperature sensor 8 a, a NOx sensor located downstream from the SCR NOx catalyst can be used to provide an input to the SCR CU that indicates the degree of effectiveness of the SCR NOx catalytic reduction process. In any embodiment in which the SCR CU 6 receives input(s) directly or indirectly indicating the temperature of the SCR NOx catalyst bed, these will provide an indication of whether or not the temperature of the SCR NOx catalyst bed should be maintained or modulated, in accordance with the SCR CU control program.

The SCR CU will be operative to control the SCR hydrogen distribution system by outputs sent to that system, e.g., through operative coupling 7 b. The SCR CU will optionally be operative to control the SCR NOx catalyst bed temperature by outputs sent to a temperature regulator, e.g., through operative coupling 8 b.

The internal RAM of the control unit 6 stores statuses of operational flags and various control data. The control unit 6 is capable of selectively controlling any of the components of the SCR hydrogen delivery system and/or SCR NOx catalyst bed temperature regulating system in accordance with the control program. It will be apparent to those skilled in the art from this disclosure that the precise structure and algorithms for the control unit 6 can be any combination of hardware and software that will carry out the functions of the present invention.

In the case of partially and fully hydrogen-fueled engines, the SCR NOx reduction system may optionally include an additional hydrogen sensor to detect the concentration of unoxidized hydrogen in the exhaust, in which case the SCR CU can be configured to operate the SCR hydrogen delivery system to introduce into the exhaust stream only the balance of hydrogen needed for NOx reduction.

The NOx reduction reactions catalyzed by an SCR NOx reduction system according to the present invention include the selective catalytic reduction of any one or more of the various NOx species present. Those most commonly found in the exhaust of diesel and internal combustion engines are NO and NO₂, and even in systems in which a NOx trap is utilized, wherein the NOx adsorption and release process can generate other species such as N₂O₄, NO⁺, and NO₃, these species commonly revert to NO and NO₂, which means that the catalytic reduction of the released adsorbed NOx species is often still performed directly on one or both of these two most common NOx species.

Thus, for purposes of selective catalytic reduction of NOx, the two chief species of importance are NO and NO₂, and these are reduced according to one or both of the following reactions, respectively: 2NO+2H₂→N₂+2H₂O   (IA) and 2NO+H₂→N₂+½O₂+H₂O   (IB); 2NO₂ +H ₂→N₂+4H₂O   (IIA) and 2NO₂+2H₂→N₂+O₂+2H₂O   (IIB). Reactions IA and IIA are favored in SCR systems. Thus the approximate molar ratio of NO:H₂ for stoichiometric reactions of NO and H₂ vary from 1:1 to 2:1, with 1:1 being favored, while the approximate molar ratio of NO₂:H₂ for stoichiometric reactions of NO₂ and H₂ vary from 1:1 to 1:2, with 1:2 being favored. Where hydrogen other than diatomic hydrogen is utilized, an equivalent stoichiometry applies.

Embodiments of the present invention may also be used with other types of combustion-based vehicle engines, preferably internal combustion engines, e.g., engines utilizing organic fuels and/or nitrogen-containing fuels. Such fuels include, but are not limited to, e.g.: hydrazine, oxyquinoline; wood, other polysaccharides or saccharides; coal, petroleum, natural gas; biological oils, resins; and fractions or other derivatives thereof; some examples of which include gasoline, kerosene, turpentine, and alcohols. Such other engines may be, e.g., 2- or 4-stroke Otto or Daimler-Otto engines. Preferably, such other engines will be those in which hydrogen is already used as an additional fuel to be mixed with the primary fuel, or those powering vehicles wherein a hydrogen supply is otherwise already available, e.g., to supply energy to an auxiliary power unit.

However, embodiments of the present invention are not limited based on the fuel used by a particular engine. Any engine in which oxidative combustion is taking place in the presence of nitrogen or a nitrogen-containing compound, either internally or externally to the power-generating chambers (e.g., cylinders or rotor chambers), can generate NOx for which SCR NOx technology can be used. Embodiments of the present invention are preferred for use in conjunction with engines that are operated under conditions in which at least significant NOx generation occurs. For example, in traditional hydrocarbon-fueled Daimler-Otto engines operated under fuel-lean/oxygen-rich conditions and high temperatures, relatively large concentrations of NOx are generated. Embodiments of the present invention are particularly useful to treat exhaust from such engines.

Advantages provided by the process and systems according to various embodiments of the present invention include the following.

Embodiments of the present invention allow hydrogen-fuel-supplemented internal combustion engines (ICE) to approach stoichiometric operation, i.e. full power, even though that generates a high concentration of NOx. In such embodiments, the hydrogen-based NOx SCR system is capable of controlling NOx in the exhaust, resulting in a low-NOx-emissions vehicle.

Embodiments of the present invention permit elimination of a separate urea reservoir or ammonia tank otherwise required for SCR of NOx. In various preferred embodiments of the present invention, where the vehicle is already equipped with an hydrogen storage unit, no further reservoir or tank for 

1. A process for selective catalytic reduction of NOx generated by an automotive vehicle engine, comprising: providing hydrogen from an on-board hydrogen storage unit of an automotive vehicle in which the hydrogen is being stored for use as a source of chemical potential energy for charging an auxiliary power unit; combining said hydrogen with exhaust gas released from the engine to form a hydrogen-exhaust gas combination; and contacting said gas combination with a selective catalytic reduction NOx catalyst under conditions in which the catalyst can catalyze the selective catalytic reduction of NOx to N₂, H₂O, and optionally O₂.
 2. An automotive vehicle system for selective catalytic reduction (SCR) of NOx generated by an automotive vehicle engine of a vehicle in which hydrogen is being stored for use as a source of chemical potential energy for charging an auxiliary power unit, the system comprising: an SCR NOx catalyst; an SCR hydrogen distribution system adapted for of delivering hydrogen from the on-board hydrogen storage unit to the exhaust gas stream at a point upstream from said SCR NOx catalyst, said hydrogen distribution system including a hydrogen injector; and an SCR control unit (SCR CU), the SCR CU being operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen directly proportional to either (1) an amount of exhaust gas generated per unit time by the engine or (2) an amount of NOx generated per unit time by the engine.
 3. The system according to claim 2, wherein said SCR system comprises a NOx sensor of adapted for sensing the concentration of NOx present in the engine exhaust, said SCR CU being operative to deliver to the exhaust stream an amount of hydrogen proportional to an input received from the NOx sensor.
 4. The system according to claim 2, wherein said SCR CU is operative to control the temperature of the SCR NOx catalyst.
 5. The system according to claim 4, Wherein said SCR system comprises a NOx catalyst temperature sensor and said SCR CU being operative to control said temperature based on an input received from the NOx catalyst temperature sensor.
 6. The system according to claim 2, wherein said SCR CU is operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen that is approximately stoichiometric with the NOx concentration in the engine exhaust.
 7. The system according to claim 3, wherein said SCR CU is operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen that is approximately stoichiometric with the concentrations of NO and NO₂ sensed in the engine exhaust by said NOx sensor, according to approximate molar ratios for NO:H₂ of from 1:1 to 2:1 and for NO₂:H₂ of from 1:1 to 1:2.
 8. The system according to claim 7, wherein said SCR CU is operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen that is approximately stoichiometric with the concentrations of NO and NO₂ sensed in the engine exhaust by said NOx sensor, according to the approximate molar ratios of 1:1 for NO:H₂ and 1:2 for NO₂:H₂.
 9. The system according to claim 2, wherein the SCR NOx catalyst comprises a metal catalyst.
 10. The system according to claim 9, wherein the SCR NOx catalyst comprises a noble metal catalyst.
 11. The system according to claim 2, wherein the SCR NOx catalyst comprises at least one of Fe₂O₃, V₂O₅, MoO₃, and WO₃.
 12. The system according to claim 2, wherein the SCR NOx catalyst comprises a zeolite catalyst.
 13. The system according to claim 2, wherein the SCR NOx catalyst is operated at a temperature that is within a range of about 180° to about 600° C.
 14. The system according to claim 10, wherein the SCR NOx catalyst is operated at a temperature that is within the range of about 180° to about 300° C.
 15. The system according to claim 10, wherein the SCR NOx catalyst is operated at a temperature that is within the range of about 230° and about 430° C.
 16. The system according to claim 12, wherein the SCR NOx catalyst is operated at a temperature that is within the range of about 360° to about 600° C.
 17. The system according to claim 2, wherein the system further comprises a NOx trap.
 18. A motorized vehicle having a fueled engine and an on-board hydrogen supply, the hydrogen being used as a source of chemical potential energy for charging an auxiliary power unit of the vehicle, wherein the vehicle further comprises a system for selective catalytic reduction (SCR) of NOx generated by the fueled engine, the system comprising: an SCR NO_(x) catalyst; an SCR hydrogen distribution system capable of delivering hydrogen from the on-board hydrogen supply to an engine exhaust stream at a point upstream of the SCR NO_(x) catalyst; and an SCR control unit operative to control the SCR hydrogen distribution system to deliver to the exhaust stream an amount of hydrogen directly proportional to either an amount of exhaust gas generated per unit time by the engine or an amount of NO_(x) generated per unit time by the engine.
 19. The motorized vehicle according to claim 18, wherein said engine is a diesel engine and said on-board hydrogen supply is operative to provide hydrogen for an on-board hydrogen-sourced auxiliary power supply.
 20. (canceled) 